CODEWITHSUNDEEP
javascriptperformance
Tim Takel
Tim Takel

Reputation: 65

When should I prefer a clone over an reference in javascript?

at the moment I'm writing a small app and came to the point, where I thought it would be clever to clone an object, instead of using a reference.

The reason I'm doing this is, because I'm collecting objects in a list. Later I will only work with this list, because it's part of a model. The reference isn't something I need and I want to avoid having references to outside objects in the list, because I don't want someone to build a construct, where the model can be changed from an inconsiderate place in their code. (The integrity of the information in the model is very important.)

Additional I thought I will get a better performance out of it, when I don't use references.

So my overall question still is: When should I prefer a clone over an reference in javascript?

Thanks!

Upvotes: 0

Views: 76

Answers (2)

siledh
siledh

Reputation: 3378

Why not just make the objects stored in the list immutable? Instead of storing simple JSON-like objects you would store closures.

Say you have an object with two properties A and B. It looks like that:

myObj = {
   "A" : "someValue",
   "B" : "someOtherValue"
}

But then, as you said, anyone could alter the state of this object by simply overriding it's properties A or B. Instead of passing such objects in a list to the client, you could pass read-only data created from your actual objects.

First define a function that takes an ordinary object and returns a set of accessors to it:

var readOnlyObj = function(builder) {
    return {
        getA : function() { return builder.A; },
        getB : function() { return builder.B; } 
    }
}

Then instead of your object myObj give the user readOnlyObj(myObj) so that they can access the properties by methods getA and getB.

This way you avoid the costs of cloning and provide a clear set of valid actions that a user can perform on your objects.

Upvotes: 0

Dave
Dave

Reputation: 46339

If stability is important, then clone it. If testing shows that this is a bottleneck, consider changing it to a reference. I'd be very surprised if it is a bottleneck though, unless you have a very complicated object which is passed back and forth very frequently (and if you're doing that it's probably an indication of a bad design).

Also remember that you can only do so much to save other developers from their own stupidity. If they really want to break your API, they could just replace your functions with their own by copying the source or modifying it at runtime. If you document that the object must not be changed, a good developer (yes, there are some) will follow that rule.

For what it's worth, I've used both approaches in my own projects. For small structs which don't get passed around much, I've made copies for stability, and for larger data (e.g. 3D vertex data which may be passed around every frame), I don't copy.

Upvotes: 2

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